The rods of a rod string which is driven at the surface, either reciprocably or rotatably, by a prime mover to operate a submerged pump at the bottom of well tubing through which the rod string extends, are subject to corrosion as is the well tubing itself by chemical and electrochemical reactions. The problems and costs involved in replacing damaged rods, damaged couplings and damaged well tubing sections and tubing section couplings are so severe that many investigations have been conducted and many programs to minimize such damage have been prepared and implemented as is documented in such articles as "Sucker Rod Failures", Oil & Gas Journal, Apr. 9, 1973 edition; "How to Mitigate Corrosion and Abrasion Rod Pump Wells", Petroleum Engineer International, May 1991 edition; "Corrosion Control Programs", Petroleum Engineer International, October and November 1985 editions. Cathodic protection by use of sacrificial anodes or by impressed direct electric current in well installations has been disclosed in such U.S. Pat. No. 2,846,385 to R. C. Buchanan, Aug. 5, 1958, and U.S. Pat. No. 3,734,181 to Donald V. Shaffer, May 22, 1973, but the structures disclosed are of limited usage and in the case of the structures disclosed in the patent to Shaffer of complex design and limited efficacy.
The well tubing and the rods of the rod strings are subjected to corrosive action of chemicals present in the formation well fluids being pumped to the surface, such as hydrogen sulfide, carbon dioxide in the presence of water, and soluble salts, and also of chemicals which may be introduced into the tubing to stimulate well fluid flow, to reduce bacterial accumulations and the like.
The damage of the well tubing and the rod string by such chemicals is most prevalent in particular temperature and pressure zones in the well tubing determined by the types and concentration of the chemicals present in the fluids in the tubing. Such zones of corrosion damage to the tubing may be determined by caliper profiling methods an of both tubing and rod string by visual inspection when the rod string and the tubing are pulled from the well.
As is described in detail in the above mentioned articles, damage minimization is sought by use of special alloy steels for the tubing section and couplings and for the rods and couplings of the rod string, and by various coatings and inhibitors for the internal surfaces of the tubing sections and tubing couplings and for the rods and couplings of the rod string.
Where such coatings are used, any voids or breaks in the coatings, whether due to improper applications of the coating or due to damage during installation or handling of these components, become locations of corrosive damage. In the case of tubing, the corrosion may cause holes to be formed through the tubing. In the case of the rods, the corrosion may cause pits to be formed which therefore become locations of mechanical weakness in the rods and of rupture of the rods under the stresses to which the rods are subjected during pumping operations. Even where no coatings are employed, the corrosive damage at locations of nicks, scrapes or abrasions on the tubing and rods is greater than at unmarred smooth surfaces of the rod string and the well tubing.